Search

This correction notice addresses some typographical errors in a paper titled "First upper limits on the 21 cm signal power spectrum from cosmic dawn from one night of observations with NenuFAR." The corrections include fixing the units for the color bar in a figure, clarifying the baselines used by LOFAR to compute the power spectrum, and adjusting the caption in another figure. These corrections are only typographical and do not impact the text, results, or conclusions of the paper. The authors of the paper are listed as S. Munshi, F. G. Mertens, L. V. E. Koopmans, A. R. Offringa, B. Semelin, D. Aubert, R. Barkana, A. Bracco, S. A. Brackenhoff, B. Cecconi, E. Ceccotti, S. Corbel, A. Fialkov, B. K. Gehlot, R. Ghara, J. N. Girard, J. M. Grießmeier, C. Höfer, I. Hothi, R. Mériot, M. Mevius, P. Ocvirk, A. K. Shaw, G. Theureau, S. Yatawatta, P. Zarka, and S. Zaroubi. [Extracted from the article]

We compare various foreground removal techniques that are being utilized to remove bright foregrounds in various experiments aiming to detect the redshifted 21 cm signal of neutral hydrogen from the epoch of reionization. In this work, we test the performance of removal techniques (FastICA, GMCA, and GPR) on 10 nights of LOFAR data and investigate the possibility of recovering the latest upper limit on the 21 cm signal. Interestingly, we find that GMCA and FastICA reproduce the most recent 2σ upper limit of |$\Delta ^2_{21} \lt $| (73)2 mK2 at k  = 0.075  h cMpc−1, which resulted from the application of GPR. We also find that FastICA and GMCA begin to deviate from the noise-limit at k -scales larger than ∼0.1  h cMpc−1. We then replicate the data via simulations to see the source of FastICA and GMCA's limitations, by testing them against various instrumental effects. We find that no single instrumental effect, such as primary beam effects or mode-mixing, can explain the poorer recovery by FastICA and GMCA at larger k -scales. We then test scale-independence of FastICA and GMCA, and find that lower k -scales can be modelled by a smaller number of independent components. For larger scales (k ≳ 0.1  h cMpc−1), more independent components are needed to fit the foregrounds. We conclude that, the current usage of GPR by the LOFAR collaboration is the appropriate removal technique. It is both robust and less prone to overfitting, with future improvements to GPR's fitting optimization to yield deeper limits. [ABSTRACT FROM AUTHOR]

A new upper limit on the 21 cm signal power spectrum at a redshift of z  ≈ 9.1 is presented, based on 141 h of data obtained with the Low-Frequency Array (LOFAR). The analysis includes significant improvements in spectrally smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally weighted power spectrum inference. Previously seen 'excess power' due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25–0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-σ upper limit of |$\Delta ^2_{21} \lt (73)^2\, \mathrm{mK^2}$| at |$k = 0.075\, \mathrm{h\, cMpc^{-1}}$| is found, an improvement by a factor ≈8 in power compared to the previously reported upper limit. The remaining excess power could be due to residual foreground emission from sources or diffuse emission far away from the phase centre, polarization leakage, chromatic calibration errors, ionosphere, or low-level radiofrequency interference. We discuss future improvements to the signal processing chain that can further reduce or even eliminate these causes of excess power. [ABSTRACT FROM AUTHOR]